(1) Field of the Invention
The present invention relates to a developing device and an image forming apparatus using the device, in particular relating to a developing device using a dual-component developer containing a toner and a magnetic carrier, for use in an image forming apparatus for forming images using the toner based on electrophotography, such as an electrostatic copier, laser printer, facsimile machine or the like, as well as to an image forming apparatus using this device.
(2) Description of the Prior Art
Conventionally, image forming apparatuses based on electrophotography such as copiers, printers, facsimile machines and the like have been known. The image forming apparatus using electrophotography forms an image by forming an electrostatic latent image on a photoreceptor drum (toner image bearer) surface, supplying toner to the photoreceptor drum by a developing device to develop the electrostatic latent image, transferring the toner image formed on photoreceptor drum by development to a sheet of paper or the like, and fixing the toner image onto the sheet by means of a fixing device.
Recently, in the image forming apparatuses that support full-color reproduction and high-quality images, a dual-component developer (which will be referred to hereinbelow as simply “developer”), which can present excellent charge performance stability, is often used.
This developer consists of a toner and a carrier, which are agitated in the developing device and frictionally rubbed with each other so as to produce appropriately electrified toner.
In the developing device, the electrified toner is supplied to a developer supporting member, e.g., the surface of a developing roller. The toner thus supplied to this developing roller is moved by electrostatic attraction to the electrostatic latent image formed on the photoreceptor drum. Hereby, a toner image based on the electrostatic latent image is formed on the photoreceptor drum.
Further, the image forming apparatus is demanded to be made compact and operate at high speed, and it is also necessary to electrify the developer quickly and sufficiently and also convey the developer quickly and smoothly.
For this purpose, in order to disperse supplied toner promptly into the developer and provide the toner with an appropriate amount of charge, a circulating type developing device is adopted in the image forming apparatus.
The circulating type developing device includes a developer conveying passage as the path in which the developer is circulatively conveyed and a developer conveying member that agitates and conveys the developer in the developer conveying passage. The developer is electrified quickly and sufficiently while it is being agitated and conveyed in the developer conveying passage by the developer conveying member (see patent document 1: Japanese Patent Application Laid-open H10-63081).
In this circulating type developing device, a toner hopper is provided on the top of the developing device, so that toner will be added from the toner hopper to the developer conveying passage when the toner concentration in the developer inside the developer device becomes lower than a predetermined level. The toner added (which will be referred to hereinbelow as “added toner”) is mixed with the previously existing developer and, electrified whilst it is being conveyed in the developer conveying passage.
In the developing device of this circulating type, however, there occurs a case where the added toner is supplied to the developing roller before the added toner is sufficiently mixed with the carrier (the previously existing developer), or in a low electrified state where the added toner has not yet been charged sufficiently.
When the toner that has not been charged sufficiently is supplied, the problem of the toner scattering from the developing roller is prone to occur, soiling the interior of the image forming apparatus and formed images. Additionally, if the toner that has not been sufficiently electrified is supplied to the developer, the electrostatic force for retaining the toner on the carrier surface is low so that there also occurs the problem that the toner tends to adhere to the non-image area and cause image fogging.
In order to deal with this problem, patent document 1 discloses a configuration where meshy screen members which the developer passes plural times are arranged in the developer conveying passage. In one example, the developer conveying member is configured of a screw that has helical feed blades around a rotary shaft while the screen members are disposed between blades.
However, even with the above developing device having screen members, a large amount of toner is added from the toner hopper to the developer conveying passage, and the toner is conveyed in a lumpy condition to the developer's top surface (developer surface) and cannot be sufficiently mixed with the carrier, thus causing the problem that insufficiently electrified toner is supplied to the developing roller.
Also, a method for enhancing the capability of mixing the added toner and the developer by making the blades of the developer conveying member larger may be considered, but this causes the problem that excessive stress is applied on the developer so that the developer is worn away quickly, shortening the life of the developer.